I have a unlocked note 10 plus and all I had to do was call AT&T support and have them add the IMEI of the phone to their system. This enabled voLTE and allowed the preloaded Voicemail app to work. You will need the IMEI number (found under About Phone>Status>IMEI info) and your SIM card number. If the voicemail asks for a password try your AT&T pin thats what worked for me.
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